The Physics of Resonance

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Antenna Demonstrations

Any piece of conductive material can function as an antenna, however antennas generally work best when they're designed to resonate at the frequency being transmitted or received. For simple antennas this is mostly a function of their length. These ideas can be tested with an inexpensive handheld radio transmitter and a compact electric field strength meter.

Cautions

 Using radio transmitters can interfere with other users. Always use the following guidelines: Keep power levels low and transmission times short. Do not allow students to play with the equipment. Listen to the radio's receiver before transmitting a signal to the transmission lines. Do not transmit if someone is using the frequency. Information on this page is provided in good faith. However, restrictions on transmitters will vary from country to country and are subject to change

The demonstrations described below can easily be used as quantitative lab experiments for high school physics classes. It is especially appropriate for AP physics classes which discuss electric and magnetic fields. When used as a lab experiment, only one radio transmitter is needed which is controlled by the teacher. Students would use the electric field strength meter to measure relative field strength.

Background
A simple monopole antenna works best when its length is sized for it to resonate. This is roughly equal to 1/4 of the wavelength it transmits or receives. See "How Antennas Work". The electromagnetic radiation emitted from such an antenna is polarized. Due to this polarity, the receiving antenna has to be oriented in the same direction for best reception.

Procedure Demo 1: Antenna Polarity
Place the Deluxe Field Strength Meter in a horizontal position with the antennas fully extended. Turn the meter to its maximum sensitivity. From a location across the room, hold a handheld radio in a horizontal position and press the transmit key. Note the reading on the Compact Field Strength Meter and adjust its sensitivity downward until the meter reads about 10% of full scale. If the meter fails to register, slowly walk forward until it reads about 10%. Keep the transmitter in a horizontal position and measure the distance to the meter. Take several meter readings along with measurements of distance as you walk forward until the meter reads 100%.

Plot the relative field strength versus distance. This should be a non-linear curve in which the field strength  strongly declines with increasing distance. The exact form is hard to predict since there are many possible sources of distortion in the electric field, not to mention possible reflections of the wave. It's best to plot this on an overhead transparency or in some other form which can be seen by the entire class.

Turn the radio transmitter to a vertical position and repeat the process described above. Compare the new readings with the old ones. They should be much lower.

This demonstration of antenna polarity can be done with a low powered FRS handheld radio. These are very inexpensive and are available from Walmart or RadioShack for as little as \$15. The demonstration can also be done with any of the radios mentioned in the "Finding a Radio Transmitter" section of the article in the Standing Electrical Waves Demonstration.

Procedure Demo 2: Optimum Antenna Length
Note: this demo has to be done with a radio whose operating frequency has a corresponding wavelength of  roughly 2 meters. In the United States, this would fit either the MURS business band or the HAM 2meter band (see "Finding a Radio Transmitter" for additional input). Otherwise, the antenna on the Deluxe Field Strength Meter will be either too short or too long.

Place both the radio transmitter and Deluxe Field Strength Meter in a horizontal position where the meter reads 100% at full sensitivity.  Reduce the meter sensitivity to about 3/4 of its maximum scale reading when the antennas of the Deluxe Field Strength Meter are fully extended. Slowly collapse the antenna and take periodic meter readings as the antenna collapses. It's best to plot this on an overhead transparency or in some other form which can be seen by the entire class. The plot should show a relative maximum around the 1/4 wavelength size. For the frequencies mentioned, this would be a length of about 0.51 meters (20 inches).

 Required Equipment Num Quantity Price Item Total Comments 1 each \$39.95 Deluxe Field Strength Meter from MFJ Enterprises.com \$39.95 This is a low cost meter. Other types are available which may work equally well. 1 each handheld radio transmitter Variable MURS business band or the HAM 2meter band is the best. (See "Finding a Radio Transmitter")
Acknowledgements:
This project was supported by a National Science Foundation Research Experience for Teachers grant as part of Clemson University's Summer Undergraduate Research Experience in Wireless Communications.

Special thanks is due to Dr. Chalmers Butler of Clemson University for his guidance and input on the preparation of this page.

For more information about wireless communication and the electromagnetic spectrum visit The Hidden World of the Electromagnetic Spectrum.

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